Data transmission systems that operate in a bandwidth-limited environment contend with the fact that constraining the bandwidth of the transmitted signal increases the likelihood of a decoding error at the receiver due to inter-symbol interference (ISI). Single-carrier communication systems often employ pulse-shaping techniques that allow for bandwidth containment while minimizing the likelihood of errors at the receiver.
In many data transmission applications, the transmitted signal is restricted to a certain bandwidth due to system design constraints or due to government regulation. Pulse shaping is a spectral processing technique by which fractional out of band power is reduced for efficient, reliable, single-carrier communication. Pulse shaping filters reduce ISI and Adjacent Channel Interference (ACI). The design and analysis of a transmit pulse shaping filter is an important aspect of digital wireless communication since it has a direct effect on spectral emissions and ISI.
Wireless communication systems, such as Wireless LAN, contend with impairments such as multipath propagation and channel fading. Multi-carrier modulation schemes, such as Orthogonal Frequency Division Multiplexing (OFDM) as employed in IEEE 802.11g/n achieve high spectral efficiency for moderate to high signal to noise ratios (SNRs) over such channels. However, many wireless systems still support more rudimentary single-carrier modulation modes for basic low data rate, low SNR (thus, high range) connectivity. In order to tackle the problems of multi-path and fading, single-carrier modulation may be coupled with a spread-spectrum technique such as the Direct Sequence Spread Spectrum (DSSS) scheme used in the IEEE 802.11b/g/n WLAN standard.
A square-root raised cosine (RRC) filter, for example, may be used as a typical pulse-shaping filter to conform to the spectral mask requirements of the IEEE 802.11b standard. Implementation of the pulse shaping filter after Barker encoding (the DSSS spreading used in the IEEE 802.11b standard) the data symbol stream and any subsequent re-sampling/up-sampling are power hungry computations in the digital transmitter baseband.
The saturation power (Psat) requirement of the Power Amplifier (PA) is another factor affecting the power consumption of a transmitter. The PA amplifies a frequency up-converted base-band signal for transmission using an antenna. Typically, the PA is the most power-hungry block in the transmitter sub-system, and the power consumption of the PA directly increases as the desired Psat increases. The Psat requirement of the PA for a particular modulation mode is in turn driven by two factors: the targeted maximum transmit power (Pmax), and the Peak-to-Average Ratio (PAR) of the base-band signal being modulated. Typically, an equation of the following form holds:Psat=Pmax+PAR+Margin
The overall Psat requirement for the system may be determined by the maximum of the requirements from each modulation mode, and typically in 802.11b WLAN, the Psat requirement from the single carrier modulation modes dominates. This implies that one way of reducing the required Psat (and hence the PA power) for a given Pmax, is to reduce the PAR of the baseband signal. Techniques to lower the power consumption and minimize the PAR of these operations may be used to solve heretofore unaddressed needs with previous solutions.